Is there any trick to obtain coordinates for a wing planform or fuse? For airfoils there's plenty of information and coordinates already created, but for a given planform or fuse it would be nice to get the numbers and insert them in Autocad.

I wonder how the guys without computers designed planes, only with french curves...?

Thanks,

Ricardo

Are you talking overall proportions, angles, etc.? When I'm working on a design, I usually start with the sizes, proportions, angles, etc and then build appearance over that. (Actually, you can get from most good designs to a replica of almost anything you want that way.)

There are some fairly standard measurements... the aspect ratio of a sport plane wing (i.e., the proportionof its length (span) to its width (chord)), is usually between 5:1 and 6:1. The wing area tends to run around a hundred square inches higher than ten times the engine size (in cubic inches -- well, actually in hundredths of a cubic inch) rounded up to the next higher size.

So, If I wanted a 62" span trainer, it would probably have a chord of about 10" (using the rule-of-thumb aspect ratio) for an area of about 610 squinches (span times chord). That should take a .40 or .46 size engine (75cc). If I want ailerons, I can cut out about 10% of the wing chord for them... so they could be 1" or 1-1/4" aileron stock. The dihedral might be 5 degrees for a 3-channel plane (no ailerons) or half of that for the 4-channel version (with ailerons).

Balance the plane at 1/4 to 1/3 of the way beack the chord... I start at 28% just to pick a reasonable number in between.

The fuselage length will be a little over 3/4 of the span -- with around 3/4 of that behind the balance point and the rest ahead. So a length of maybe 46". The stab and elevator will be about a quarter of the wing span for 150 squinches (of which about a quarter is elevator)... the fin & rudder add up to a third of the size of the stab and elevator (and a third of that is rudder).

It's easy to start the design with a "semi-symmetrical" airfoil like the venerable NACA2412. (It's been with us for probably 70 years or more, but hold it up to the light withsome of the best of today's airfoil profiles and you'll be hard pressed to detect the difference.) Sitting such an airfoil flat on the wing saddle will give it a 2 degree positive incidence relative to a level referrence line the length of the plane. That ain't bad. Set the stab parallel to the referrence line and point the engine down two degrees and you have a pretty solid, well-proven setup.

There are a lot of articles around (including on the web) with recipes like this for a successful starting design -- they'll vary a little one from the other, but all will be simmilar. I think Howard Sullivan's R/C Dlight Unlimited sight has such an article and I know it's goit a lot of other great stuff... read it all. Most of what we do in the design arena is far from new, so there are many good references and lots of peolpe to give advice.

My problem is how can I draw, say, the wing of the Bird of Time, that has a very singular shape if you look the top of the wing.

In wings double tapered and round tips it's dificult to draw by hand in a CAD program.

The same for the fuse.

Ahhhh, I see.

The easiest thing is if you have a drawing of the plan view of the Bird, you could take some quick measurements of the straight parts and use a CAD curve fitting function to do the curves (like at the tips and where the outer panes start). If you don't happen to have a wrong-sized plan of the plane, I can probably scan you one and get it to you... The BoT was published in RCModeler, so I probably have the article with the miniature plan. And if I don't have the article, I at least have a copy of the miniature in the plans digest.

Really, the very easiest is to buy a copy of the plan -- available from R/C Modeler -- or enlarge from a miniature. The plan is not expensive, but it does sometimes take a long time for you to receive it.

If you are like me, the very easiest is not what you really desire. You would probably like to do the Bird of Time, but you would also like to know how to do things for use on other planes. Is that a good guess?

Yes, that's the point. I can scan a picture, load it in a CAD program and draw over it, sure it works. BUT, if I use Compufoil to draw the wing, I think I'll need the wing coordinates.

Thank you very much for your time.

Well, for a lot of planes (Bird of Time is one of them) the discoverer doesn't give a standard airfoil that we could look up coordinates for. (For the BoT, Ol' Buzzard tells us that he built 12 of them before he arrived at the published version, that the idea behind its airfoil was to use a thin profile. He tried sections as low as 8%, but he said he used 9% on the plans because he found no performance improvement by going down to 8% and he could build a stronger wing at 9%. He also talks Phillips entry, so he may have started with something standard, say NACA 2409 and effectively increased the camber by raising the entry point.... effectively reinventing something very close to a NACA 3409 (which is essentially a flat-bottomed foil with phillips entry and overall proportions to match the BoT.

So, I would start by printing out a 3409 and an enlargement of the Bird of Time airfoil and see how close they are... you can adjust the NACA foil to try and match the BoT one and you may just get exactly what you're after.

If not, one thing you could do load a scan into a CAD program as you mentioned, but rather than drawing the curves manually, use a minimum number of points and have the spline function of the CAD program fit a curve to them. If the points are placed well and the tangent angles at the end points are locked in parallel to those of the original foil, a very good match should be generated with just a few splines (and so with just a few points defined). It is best to keep the number of points to a minimum and adjust the shape of the curve by moving points along the desired shape that you get from the scan. Once you have a good fit this way, you can throw a set of verticle lines over the airfoil and pluck off the coordinates at the points where they intersect the airfoil curve.

I'd bet that some of the CAD whizards know quicker methods of pulling off coordinate data, but the basic method should work.

Do you have a setup you can experiment with? A CAD program loaded and maybe a scanned plan of the Bird of Time?

I have Auto CAD Lite. I'm going to search a wing to scan or a fuse and paste it in ACAD and play with splines.

For splines I went to a AutoCad Yahoo groups discussion group for hints and... no, no hints in splines were provided. An ACAD guru said practice and practice. The ACAD manual isn't very clear in this subject, so I think I have to waste some time taming splines.

Thank you for all your help.

I went to school for computer science, so going through the process of computing splines from a few points was one of the things we learned to do. Something we learned was that as long as the curve you want doesn't do any radical changes in curvature, it can be modeled by a single spline (which means two end poins, a well-chosen point in between, and both ends "clamped" at the proper angle).

It's interesting to take a scanned curve, lay tangent lines at each end of the main part of the curve to use for a guide(like with ends at the trailing edge and a small distance behind the leading edge) , then define a spline with just the end points and one point along the airfoil in between... then watch how you can change the spline by moving that mid point along the airfoil curve.

It's also interesting to see how close most airfoils come to the NACA standard curves... I think the 4-digit NACA series is generated using a single spline curve for most of the curve and a constant radius for the leading edge, so it makes sense that a good approximation may use one or two mid points... one for the spline representing the main part of the surface and one for the spline representing the leading edge radius. (A spline defined right can exactly match a part of a circle or a part of any other conic section. They do great things!)